1. Field of the Invention
The invention relates to a method of reactively sputtering a transparent, electrically conductive metal oxide film or coating on a substrate surface, e.g. the surface of a glass sheet.
2. Description of the Prior Art
It is known to produce such coatings on glass substrates but it is constantly desired to be able to produce a coating which has the maximum light transmission and the minimum colour, as well as a resistance to abrasion and a low electrical resistance. In practice, it has been usual to employ a gold coating which has an acceptable light transmission of 70% and a low electrical resistance of 10 ohm/sq., but has a strong yellow colour and a poor resistance to abrasion.
Coatings of a large variety of metal oxides have also been proposed, including indium based alloys, e.g. an indium/tin alloy. One such proposal includes sputtering a substrate in a vacuum chamber having an oxygen containing atmosphere, and subsequently heating the sputtered coating to at least 200.degree. C in a non-oxidising atmosphere to lower its electrical resistance. This proposal thus relates only to a two-stage process which is necessary since the sputtered coating, without being subsequently heated, does not possess a sufficiently low electrical resistance. The quantity of oxygen in the atmosphere of the vaccum chamber is maintained relatively high to obtain an oxide coating with an acceptable light transmission, since before now, whilst it has been known that a reduction of the oxygen content will increase the electrical conductivity of the sputtered coating, it has been believed that such a reduction must essentially be accompanied by the production of substantially a pure metal coating with an unacceptable degree of light transmission. The necessity for post-heating is disadvantageous since it incurs a second operating step and a longer period of time to produce a coating having the desired physical characeteristics. Also the step of post-heating is believed to be detrimental to the stability of the resultant coating, leading to a deterioration, i.e. an increase, of the specific electrical resistivity of the coating.
Another previous proposal avoids the step of post-heating the sputtered film by sputtering the coating in a vacuum chamber having an atmosphere of both oxygen and hydrogen. As before, it has been believed necessary to maintain a relatively high quantity of oxygen. More particularly, it is most undesirable for the atmosphere to contain hydrogen because of the risk of explosion.
Many other proposals have been made for the sputtering of these coatings but none of them are believed to fulfil an object of the invention which is to reactively sputter a metal oxide coating in an atmosphere containing a relatively small amount of oxygen compared with the quantities used before now, whilst obtaining a directly sputtered coating which is transparent, haze-free and has a low specific electrical resistivity which so far as possible remains stable over an extended period of time. Such a process does not require any post-heating step or an atmosphere containing hydrogen.